Its dead easy.... once you know how.
1. Get the commented config from Bis. Cut and paste the section on JeepMG into your own config.
2. Create your model in O2. Notice from your config you have to create a turret and a MG, so you need to add things like osaHlavne, usti konec and stuuf like that into your memory LOD.
3. If you don't know what the above means you are going to have to play around with O2. Try getting hold of the demo models - there is a link to them in this post
http://www.flashpoint1985.com/cgi-bin/ikonboard301/ikonboard.cgi?s=3d96d9e03e54ffff;act=ST;f=47;t=20037 or d/l some armour addons that you see floating around and look at them.
4. Even if all this looks way too complicated - do not loose hope. My first ever model - a beach buggy with 4 wheels, about 4 other faces was made in about 2 hours. So things are possible if you work at it.
5. You can only put MGs on cars. It is not possible to put any heaver weapons on them, so all those wanting to make armoured cars/LAV25's/SdKfz 234/2 are up the creek without a paddle